CHAPTER XLVIII. 

 NUTRITIVE HISTORY OF CARBOHYDRATES AND FATS. 



The Carbohydrate Supply of the Body. The available carbo- 

 hydrate material of the body consists of the glycogen found in the 

 tissues, especially in the liver (1 to 4 per cent, or more) and mus- 

 cles (0.5 per cent.), and the sugar formed from this glycogen and 

 present constantly in the blood to the amount of 0.1 to 0.15 per 

 cent. In addition it is believed that during starvation glycogen 

 or sugar may be made from the proteid tissues of the body, and 

 possibly also from the body fat, although this latter source is 

 doubtful. The supply of glycogen under normal conditions is main- 

 tained chiefly by the carbohydrate food. As was explained in the 

 section on digestion, the starches, sugars, gums, etc., which con- 

 stitute the carbohydrate foodstuffs, are eventually absorbed into the 

 blood as simple sugars, chiefly dextrose, but probably also some 

 levulose and galactose. These simple sugars constitute the important 

 glycogen formers. There is still some difference of opinion as to 

 whether all proteids are capable of yielding glycogen to the body. 

 Some physiologists believe that after the nitrogen is split off to form 

 urea, the non-nitrogenous portion of the molecule may be con- 

 verted to glycogen in the liver. Others hold that only those pro- 

 teids, such as egg-albumin, which contain a carbohydrate grouping 

 in the molecule are capable of yielding glycogen in the body.* 

 Whether the fats of the food may serve as a source of glycogen is 

 also an open question, but the balance of evidence is probably 

 against such a view. The store of glycogen in the body is about 

 equally divided between the liver and the muscular tissues, but the 

 regulation of the supply of sugar to the blood is usually attributed 

 to the liver. This regulation is adjusted so that the percentage of 

 sugar in the blood is kept astonishingly constant, not only during 

 the conditions of ordinary living, but under such an abnomal con- 

 dition as prolonged starvation. It is assumed that this regulation 

 is effected mainly by an enzyme formed in the liver cells which 

 converts the glycogen to dextrose in proportion as the sugar of the 

 blood is used up by the tissues. 



The Final Fate of the Carbohydrate of the Body. Eventually 



* See Pfluger, in " Archiv f. die gesammte Physiologie, " 96, 1, 1903, for 

 literature and discussion. 



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